1. Field of the Invention
The present invention relates to a liquid crystal display device for use in a display for office automation equipment, computers, television receivers, or the like as a substitute for a cathode ray tube.
2. Description of the Prior Art
CRTs (cathode ray tubes) have so far been widely used as a display in the peripheral equipment of office machines, computers, and so on. But nowadays liquid crystal display devices are progressively replacing the CRTs because they can be miniaturized, consume smaller power, and are obtainable at low cost.
FIG. 1 is a sectional view of a prior art liquid crystal display device disclosed, for example, in Japanese Laid-open Patent Publication No. 60-229089 and FIG. 2 is a perspective view of the same. Referring to the figures, reference numeral 1 denotes a liquid crystal display panel composed of two glass plates 2, 3 with a transparent conductive film formed thereon, a sealant 4 for joining the glass plates 2, 3 together at their peripheral portions, liquid crystals 5, and polarizing plates 6 stuck onto the front and the back of the glass plates 2, 3. In the case of an active matrix display, thin-film transistors are formed on the glass plate 3, and of a color display, a color filter is formed on the glass plate 2 or 3. Reference numeral 7 denotes a drive IC for supplying an electric signal necessary for the liquid crystal display panel 1, and 8 denotes a protection coating for moisture proofing and mechanical protection and reinforcement of the drive IC 7. Reference numeral 10 denotes a circuit board having a plurality of drive ICs mounted thereon and having a conductive pattern providing a drive circuit for the liquid crystal display panel formed thereon. A flexible printed board formed of a polyimide film is normally used for the circuit board 10 and it is provided with a reinforcement lining plate 9 stuck onto its back side. As another prior art arrangement, there is such as disclosed, for example, in Japanese Laid-open Patent Publication No. 62-238684, in which a plurality of film carriers with a drive IC connected thereto are coupled with a rigid printed board with a conductive pattern serving as a drive circuit provided thereon.
For connecting the circuit board 10 with the liquid crystal display panel 1, such methods are used as soldering, thermocompression bonding using an anisotropic conductive film and so on, mechanical pressure joining, or the like. On a transparent conductive film 11, there is provided Ni or Ni/Au coating, pretinning or the like, according to the need. The circuit boards 10a, 10b, 10c connected with each side of the liquid crystal display panel 1 are interconnected with each other by means of a flexible printed board, a flat cable, wire bonding, or the like. Reference numeral 12 denotes a light box casing fixed to the back side of the glass plate 3 and contains a lamp 13, a reflecting plate 14, and a light diffusing plate 15.
Since the liquid crystal display device organized as described above has the circuit board 10, together with the reinforcement lining plate 9, stuck to the periphery of the liquid crystal display panel 1, it becomes larger in size than the liquid crystal display surface by the width of the circuit board 10 disposed at the periphery, and therefore miniaturization of the device cannot be attained. Further, as the liquid crystal display becomes larger in size, the flexible printed board of a polyimide film tends to produce large changes in its dimensions by the effects of temperature and moisture, causing the terminals of the circuit board 10 to deviate from the positions for connection with the liquid crystal display panel 1 at each edge thereof and making the alignment difficult. In the case where a plurality of film carriers are used, the number of times of alignment performed for terminal connection and the number of connections increase, and therefore, a longer processing time is required and automatic assembling becomes difficult.
In order to improve quality of images and fineness of patterns displayed on the liquid crystal display device, there has been developed a liquid crystal display device of an active matrix type with switching elements of thin-film transistors arranged in a matrix. FIG. 3 is a partial plan view of terminal portions to be connected with a drive circuit module of a liquid crystal display panel incorporating such thin-film transistors therein. Referring to the figure, reference numeral 1 denotes the liquid crystal display panel, 3 denotes an array substrate formed of a glass plate with thin-film transistors provided thereon, 2 denotes a counter substrate formed of a glass plate with a transparent electrode provided all over its surface, and 4 denotes a seal portion for joining the array substrate 3 and the counter electrode 2 together, thereby forming them into a liquid crystal cell. Reference numeral 21 denotes a group of transparent electrode terminals led out from the source electrodes of the thin-film transistors and reference numeral 22 denotes a group of transparent electrode terminals led out from the gate electrodes of the same. While one group of transparent electrode terminals 21 are connected with a group of output terminals of a drive circuit module with a plurality of ICs for image data processing mounted thereon, the other group of transparent electrode terminals 22 are connected with a group of output terminals of another drive circuit module with a plurality of ICs for linear scanning circuits mounted thereon.
FIG. 4(a) and FIG. 4(b) show an example of connection of a liquid crystal display panel with drive circuit modules in an active matrix liquid crystal display device disclosed in Japanese Laid-open Patent Publication No. 60-229090. Denoted by 1 to 4 are the same parts as those denoted by the same reference numerals in the liquid crystal display panel shown in FIG. 3. Reference numeral 5 denotes liquid crystals sandwiched in between the array substrate 3 and the counter substrate 2, 10 denotes a circuit board made of ceramic, glass, glass epoxy, glass polyimide, or the like, 7 denotes an IC chip mounted on the circuit board 10, 23 denotes a bonding wire connecting the circuit board 10 with the IC chip 7, and these parts together constitute a drive circuit module 18. The transparent electrode terminals (not shown) formed on the array substrate 3 are provided with Au coating or the like in advance, and the drive circuit module 18 is connected with the array substrate 3 by means of wire bonding 28. Further, the drive circuit modules 18a, 18b, 18c, and 18d are interconnected with each other by means of wire bonding, a flat cable, or a flexible printed board (all of which are not shown).
Since the active matrix liquid crystal display device organized as described above has the configuration in which the drive circuit module 18 is fixedly stuck onto the array substrate 3, the array substrate is required to have a size large enough to provide the region on which the drive circuit module 18 is fixed in addition to the area in which the groups of the transparent electrode terminals are disposed. Thus, there has been a problem that the external size of the device becomes too large for the display area. Further, the ICs for image data processing and the ICs for linear scanning circuits, mounted on the drive circuit modules 18 for supplying signals to the opposing groups of the transparent electrode terminals 21 or 22 on the source side or the gate side, are each of the same function but the positions of the output terminals are inverted, and therefore, chips in which output sequences rotate in the opposite directions, so-called mirror chips, are required. Thus, it is required to change the patterns on the circuit board 10 and the programs used for automatic IC bonding, and further, the drive circuit modules 18 must be classified according to their uses. Efficiency of the assembly work has therefore been impaired.
As a measure to solve such problems, it is considered to arrange the groups of terminals on the circuit board 10 constituting the drive circuit modules 18 to be common for all the four sides of the circuit board or to be common for the upper and lower two sides and for the left and right two sides of the circuit board, as in the prior art liquid crystal display device shown in FIG. 1 and FIG. 2. Then, for example, the drive circuit modules in common can be used for image data processing circuits at the upper and the lower sides of the liquid crystal display panel and the drive circuit modules in common can be used for linear scanning circuits at the left and the right sides of the liquid crystal display panel. Thus, unification of the used ICs and improvement in the yield rate in the assembly work can be expected. However, since the groups of the terminals connected with the array substrate 3 are provided at the opposing two edges of the periphery of the substrate, the size of the circuit board becomes still larger than that in the liquid crystal display device shown in FIG. 4, and therefore, the array substrate 3 for having the drive circuit module 18 fixedly stuck thereto must also be made still larger, thus hindering the miniaturization of the device. Further, there has been a problem that the number of wires for forming the groups of terminals at two sides becomes very large, whereby multilayer wiring have had to be provided.